In chromatography, a test fluid in which a chemical sample containing numerous substances is dissolved is passed through a column of packing material contained in a tube. Because different substances in the sample have different affinities for the packing material, the time at which each substance emerges from the column will vary. The presence of a substance in the output fluid is generally detected by measuring changes in the physico-chemical properties of the fluid. A plot of these properties versus time will exhibit response peaks corresponding of each of the substances.
To improve resolution between response peaks, it is desirable to provide uniform flow paths for the test liquid through the column. Irregular-sized voids between particles in the packing material allow portions of the test liquid to advance ahead of others, thereby defeating the separation performed by the column and leading to an overlapping of the responses, so that resolution between response peaks is lost. Irregular-sized voids can occur within the packing and also at the interface between the packing and the tube wall.
The co-pending U.S. patent application of McDonald et al., Ser. No. 848,752, filed Nov. 4, 1977 (a continuation of Ser. No. 638,301, filed Dec. 8, 1975), entitled "Radial Compression of Packed Beds," both abandoned discloses eliminating irregular voids by radially compressing the packing material. Further, it discloses that voids between the packing material and the tube wall can be eliminated by deforming a plastic wall against the packing. The application discloses a variety of methods and apparatus for achieving radial compression and wall deformation. These include initial radial compression at the time of filling and further compression during use; using a plastic cartridge preexpanded with gas internally while filling and then compressed with gas externally during use; using a steel cartridge preexpanded by heating and pressurized slurry filling to provide permanent radial compression on contraction; using external compression pressures of between 10 and 1000 psi; and using cartridges with walls between 1 and 100 mils thick.
Another cause of degraded peak resolution is poor radial distribution of incoming and outgoing fluid. With cartridges that have substantial transverse dimensions relative to the size of the particles of the packing material (although the transverse dimension is generally much less than the longitudinal dimension), it is necessary to create a radially uniform flow profile through the cartridge. Simply allowing the fluid to enter and leave through small openings along the longitudinal axis causes overlapping of response peaks because fluid traveling along the longer radially outward paths takes a longer time to traverse the column than fluid traveling along the centerline.
There are commercially-available cartridges which provide radial distribution. A preparative chromatography cartridge (30 cm.times.5 cm) sold by Waters Associates, Inc., uses a flat transverse plate with radially spaced holes; the holes communicate with a funnel chamber which moves the fluid between inlet and outlet conduits and the hole locations. A cartridge sold by Unimetrics Corporation under the Knauer trademark employs a thin plate inside the cartridge with radially-extending slits grouped in patterns of three so as to have the appearance of a bird footprint. Fluid moves radially in the slits while also flowing longitudinally through an abutting filter screen and then through a succession of fiber glass filters before reaching the packing material. In practice, mixing occurs in the series of filters, and peak resolution is thus degraded.
While both these cartridges distribute the fluid radially, neither they nor any others known have adequately eliminated another source of mixing, namely, regions of excessive volume within the distributor or the filter assembly. This excessive volume causes what is known as "outer column peak spreading" in which backmixing of separated substances occurs outside the column, resulting in poor resolution of response peaks.
A further source of degraded resolution is clogging of the filters provided at the inlet of the cartridges to retain the packing material. They can become clogged with foreign particles suspended in the test fluid. The above-mentioned Knauer cartridge can have its filter replaced but to do so requires removal of the cartridge retaining cap.